This invention relates to a method for preparing a lithiated manganese dioxide having a stabilized xcex3-MnO2-type crystal structure wherein a nominally dry mixture of a manganese dioxide powder and a lithium salt are mechanically activated prior to heat-treatment. The invention also relates to the application of said prepared lithiated manganese dioxide as an active cathode material in a primary lithium electrochemical cell.
Manganese dioxides suitable for use in lithium primary cells include both chemically produced manganese dioxide known as xe2x80x9cchemical manganese dioxidexe2x80x9d or xe2x80x9cCMDxe2x80x9d and electrochemically produced manganese dioxide known as xe2x80x9celectrolytic manganese dioxidexe2x80x9d or xe2x80x9cEMDxe2x80x9d. CMD can be produced economically and in high purity, for example, by the methods described by Welsh et al. in U.S. Pat. No. 2,956,860. Typically, EMD is manufactured commercially by the direct electrolysis of a bath containing manganese sulfate dissolved in a sulfuric acid solution. Manganese dioxide produced by electrodeposition typically has high purity and high density. Processes for the manufacture of EMD and representative properties are described in xe2x80x9cBatteriesxe2x80x9d, edited by Karl V. Kordesch, Marcel Dekker, Inc., New York, Vol. 1, 1974, pp.433-488.
Typically, EMD is composed of a xe2x80x9cgamma(xcex3)-MnO2xe2x80x9d phase having a complex crystal structure consisting of an irregular intergrowth of predominantly xe2x80x9cramsdellitexe2x80x9d-type MnO2 phase and a smaller portion of xe2x80x9cpyrolusitexe2x80x9d or beta(xcex2)-MnO2 phase as described by deWolfe (Acta Crystallographica, 12, 1959, pp.341-345) and by Burns and Burns (e.g., in xe2x80x9cStructural Relationships Between the Manganese (IV) Oxidesxe2x80x9d, Manganese Dioxide Symposium, 1, The Electrochemical Society, Cleveland, 1975, pp. 306-327), incorporated herein by reference. Disorder in the crystal lattice of the xcex3-MnO2 phase can include various non-coherent lattice defects, for example, stacking faults, micro-twinning, Mn+4 cation vacancies, Mn+3 cations from reduction of Mn+4 cations, inserted protons (i.e., hydrogen ions), lattice distortions introduced by the presence of Mn+3 cations (i.e., Jahn-Teller effect) as well as compositional non-stoichiometry as described, for example, by Chabrxc3xa9 and Pannetier (Prog. Solid State Chem., Vol. 23, 1995, pp. 1-130) and Ruetschi and Giovanoli (J. Electrochem. Soc., 135(11), 1988, pp. 2663-9), incorporated herein by reference.
Electrochemical manganese dioxide is the preferred manganese dioxide for use in primary lithium cells. One consequence of the electrodeposition process is that EMD typically retains surface acidity from the sulfuric acid of the electrolysis bath. This residual surface acidity must be neutralized, for example, by treatment with a solution of aqueous base, before the EMD can be used as the active cathode material in primary lithium cells. Suitable aqueous bases include: sodium hydroxide, ammonium hydroxide (i.e., aqueous ammonia), calcium hydroxide, magnesium hydroxide, potassium hydroxide, lithium hydroxide, and combinations thereof. Typically, commercial EMD is neutralized with a strong base such as sodium hydroxide because it is highly effective and economical. However, before the neutralized EMD can be used, it must be heat-treated to remove residual water. The term xe2x80x9cresidual waterxe2x80x9d, as used herein includes surface-adsorbed water, noncrystalline water (i.e., water physisorbed or occluded in pores) as well as water present in the crystal lattice in the form of protons. Heat-treatment of EMD prior to use in lithium cells is well known and has been described, for example, by Ikeda et al. (e.g., in xe2x80x9cManganese Dioxide as Cathodes for Lithium Batteriesxe2x80x9d, Manganese Dioxide Symposium, Vol. 1, The Electrochemical Society, Cleveland, 1975, pp. 384-401), incorporated herein by reference.
EMD suitable for use in primary lithium cells can be prepared by heat-treating commercial EMD at temperatures between about 200 and 350xc2x0 C. as taught by Ikeda et al. in U.S. Pat. No. 4,133,856. This reference discloses that it is preferable to heat-treat the EMD in two steps. The first step can be performed at a temperature greater than about 100xc2x0 C., but below about 250xc2x0 C. in order to drive off surface and non-crystalline water. The second step is performed at between about 250 and 350xc2x0 C. to remove the lattice water. This two-step heat-treatment is disclosed to improve discharge performance of primary lithium cells because surface, non-crystalline, and lattice water can be removed. An undesirable consequence of the heat-treatment is that the xcex3-MnO2-type structure is gradually converted to a gamma/beta (xcex3/xcex2)-MnO2-type structure at temperatures  greater than 350xc2x0 C. The term xe2x80x9cgamma/beta-MnO2xe2x80x9d as used in the art reflects the fact that a significant portion of xcex3-MnO2 (i.e., the ramsdellite-type MnO2 phase) is converted to xcex2-MnO2 during heat-treatment. At least about 30 percent by weight and typically between about 60 and 90 percent of the ramsdellite-type MnO2 phase is converted to xcex2-MnO2 during conventional heat treatment as taught, for example, in U.S. Pat. No. 4,921,689. The produced xcex3/xcex2-MnO2 phase is less electrochemically active than EMD containing predominantly ramsdellite-type MnO2. For example, cathodes containing EMD enriched in xcex2-MnO2 are disclosed in U.S. Pat. No. 5,658,693 to provide less lithium uptake capacity during discharge in lithium cells.
A method for preparing an improved manganese dioxide from commercial lithium grade EMD having a xcex3-MnO2 structure that is not converted appreciably to the xcex3/xcex2-MnO2 structure by a heat-treatment of the type described hereinabove is disclosed in co-pending commonly assigned U.S. application Ser. No. 09/496,233, filed on Feb. 1, 2000. In the method disclosed in this reference, commercial lithium grade EMD is treated with a liquid source of lithium cations by a process that promotes stepwise ion-exchange of mobile hydrogen ions in the xcex3-MnO2 crystal lattice and on the surface of the EMD particles with lithium cations followed by heat-treatment to eliminate residual water. After such heat-treatment, typically less than about 5 wt % of additional xcex2-MnO2 phase can be detected by x-ray powder diffraction analysis. In one aspect of the disclosed method, a suspension of commercial lithium grade EMD in water is treated with a basic lithium salt, such as lithium hydroxide, by a multi-step process in which the lithium salt is added incrementally with soaking periods between additions until a pH value between about 10 and 13 is obtained. During the soaking periods, hydrogen ions in the EMD crystal lattice can ion-exchange with lithium cations to form a lithium ion-exchanged intermediate product. This intermediate product is separated from the suspension, dried, and heat-treated. In another aspect of the disclosed method, an essentially dry mixture of commercial lithium grade EMD and a low melting point lithium salt, such as lithium nitrate, is heat-treated initially at a temperature above the melting point of the salt but less than about 300xc2x0 C. and subsequently at a temperature greater than about 350xc2x0 C., but less than about 420xc2x0 C. The lithiated manganese dioxide products obtained by both processes were disclosed to have a stabilized xcex3-MnO2-type crystal structure and to provide improved performance when used in primary lithium cells. Specifically, the average operating voltage of such cells at high discharge rates and low operating temperatures was disclosed to be substantially higher than that of primary lithium cells containing heat-treated manganese dioxide of prior art not treated stepwise with a liquid source of lithium cations prior to heat-treatment.
In addition to the prior art methods disclosed hereinabove, another method for preparing lithium manganese composite oxides having the formulas Li0.33MnO2 and LiMn2O4 for use in lithium ion rechargeable cells is disclosed by U.S. Pat. No. 5,911,920. The method disclosed in the reference includes pulverization and mixing of a lithium source compound such as lithium oxide, lithium hydroxide, lithium carbonate, and the like, with a manganese source compound such as manganese dioxide, manganese oxyhydroxide, and the like in a pre-determined mole ratio under an inert gas atmosphere for a sufficient period of time (e.g., 1-30 hours) to form a lithium manganese oxide composite. It was disclosed that after the period of mixing and pulverization, no x-ray diffraction peaks characteristic of either reactant could be detected for the mixture. The mixture was next heat-treated at pre-determined temperatures (e.g.,  less than 350xc2x0 C. for Li0.33MnO2, 450-750xc2x0 C. for LiMn2O4) to crystallize product phases and remove residual water. It also was disclosed that cathodes containing the heat-treated lithium manganese oxide composites had discharge capacities in lithium ion cells comparable to cathodes containing similar compositions produced by other methods of prior art.
Another method for preparing the lithium manganese oxide LiMn2O4 having a spinel structure via mechanochemical synthesis was reported by N. V. Kosova et al. (Journal of Solid State Chemistry, Vol. 146, 1999, pp. 184-8) and G. P. Ereiskaya et al. (Inorganic Materials, Vol. 32, no. 9, 1996, pp. 988-90; translated from Neorgan. Material., Vol. 32, No. 9, 1996, pp. 1121-30). This method includes high-energy milling or grinding of a mixture of xcex2-MnO2 and a lithium salt such as lithium hydroxide followed by heat-treatment. In contrast to the observations disclosed in U.S. Pat. No. 5,911,920 cited hereinabove, Kosova et al. reported no substantial changes in the x-ray diffraction patterns of reactant mixtures after 10 minutes of high-energy milling other than a general broadening of the peaks and a decrease in peak intensities due to a decrease in particle size and some amorphization. Also, no new peaks were observed before heat-treatment. Ereiskaya et al. concluded that such a mechanical activation process can accelerate the rate of spinel phase formation at temperatures between 150 and 500xc2x0 C. well below the typical reaction temperature of xe2x89xa7650xc2x0 C. Kosova et al. suggested that during high energy milling of xcex2-MnO2 and LiOH, a redox reaction involving reduction of Mn+4 cations by OHxe2x88x92 anions can form Mn+3cations and promote diffusion of lithium cations into the particles. It was also concluded that the use of a lithium compound having hydroxyl groups is required since no chemical interaction was observed after high energy milling of xcex2-MnO2 and Li2CO3. These latter results are clearly in contradiction with results obtained by the method of the present invention.
A method for the mechanochemical synthesis of a lithium manganese oxide LiMnO2 having a rocksalt-type crystal structure by high-energy ball milling of a mixture of lithium oxide, EMD, and manganese metal powders for extended periods (i.e., up to 48 hours) in an argon atmosphere has been reported by M. N. Obrovac et al. (Solid State Ionics, Vol. 112, 1998, pp. 9-19). However, discharge performance of cathodes containing the LiMnO2 product in rechargeable lithium cells was reported to be poor.
Thus, even though considerable effort has been expended, as evidenced by the prior art cited hereinabove, the methods used to prepare lithium manganese oxides require additional refinement in order to improve substantially their performance when employed as active cathode materials for primary lithium cells.
It is a principal object of the invention to produce a lithiated manganese dioxide that results in improved discharge performance of lithium primary electrochemical cells when the lithiated manganese dioxide is employed as the active cathode material therein.
A principal aspect of the method of the present invention is directed to forming an essentially dry reaction mixture comprising manganese dioxide and a lithium salt and subjecting the reaction mixture to a mechanical activation process. Particulate rigid milling media are preferably added to the reaction mixture prior to mechanical activation. The manganese dioxide has a xcex3-MnO2-type crystal structure, and can be, for example, an electrolytic manganese dioxide (EMD) or a chemical manganese dioxide (CMD). The reaction mixture, preferably with milling media therein, is subjected to mechanical activation to promote the ion-exchange of protons (viz., hydrogen ions) located in the xcex3-MnO2 crystal lattice (as well as on the surface of the MnO2 particles) by lithium cations. During mechanical activation, a lithiated manganese dioxide intermediate product having a xcex3-MnO2-type crystal structure is formed. The intermediate product has the formula LixMnO2, wherein, 0.05xe2x89xa6xxe2x89xa60.125. The intermediate product is separated from the milling media and is subjected to a heat-treatment to remove residual water and form a lithiated manganese dioxide final product having a predominantly xcex3-MnO2-type crystal structure and the formula LiyMnO2-xcex4, wherein 0.05xe2x89xa6yxe2x89xa60.175 and 0.01xe2x89xa6xcex4xe2x89xa60.06. The lithiated manganese dioxide final product can desirably be included in the cathode of a lithium primary electrochemical cell.
The term xe2x80x9cessentially dryxe2x80x9d as used herein shall be understood to include the possibility of residual water being present in the manganese dioxide and/or water of hydration associated with the lithium salt as well as water physisorbed by the reaction mixture. The term xe2x80x9cresidual waterxe2x80x9d as used herein in connection with the MnO2 or the lithiated manganese dioxide intermediate product shall be understood to include water such as lattice water from unexchanged protons and also water in closed or open pores.
Further, the process of the present invention seeks to improve the lithiated manganese dioxide in a manner that preserves the total concentration of Mn+4 cations. This can be accomplished by inserting lithium cations into the manganese dioxide crystal lattice predominantly via ion-exchange with protons rather than reductive insertion by lithium from the lithium salt during heat-treatment, which can produce undesirable reduction of Mn+4 cations.
Thus, in one aspect of the invention, an essentially dry mixture of MnO2, known in the art as xe2x80x9clithium gradexe2x80x9d EMD or CMD, and a lithium salt, such as lithium hydroxide or lithium carbonate, is subjected to high-efficiency milling or grinding with small milling media at ambient temperature in air for a period of time ranging from about 0.25 to 8 hours followed by heat-treatment in air for about 4 to 12 hours at between 350 and 420xc2x0 C. to remove residual water. The heat-treated lithiated manganese dioxide product can include from about 0.5 to 1.5 wt % lithium, preferably from about 0.75 to 1.25 wt % lithium.
It is a further object of the present invention to provide an electrochemical cell including a cathode containing the lithiated manganese dioxide prepared by the method of this invention, an anode, and an electrolyte having improved discharge performance. The anode can be lithium metal, a lithium alloy such as lithium aluminum alloy or a lithium-insertion compound. The electrolyte can be a liquid or a polymeric electrolyte. A suitable liquid electrolyte can be a solution containing an electrochemically stable lithium salt such as lithium trifluoromethanesulfonate dissolved in suitable organic solvents such as ethylene carbonate, propylene carbonate, 1,2-dimethoxyethane, and mixtures thereof.
The lithiated manganese dioxide prepared by the method of the present invention provides the following advantages. When included as the active cathode material in a primary lithium cell, the lithiated manganese dioxide provides higher average operating voltage compared with primary lithium cells of prior art when discharged at high rate, at low temperature, and especially at both high rate and low temperature. Accordingly, lithium primary cells containing the lithiated manganese dioxide of this invention are particularly useful for demanding high performance applications, for example, use in compact photographic cameras, digital cameras or digital video camcorders.
Other features and advantages of the invention will be readily apparent from the description of the preferred embodiments and from the claims.